Recent data from my laboratory demonstrates a novel 6kb mRNA is present in a human colon polyp cell line, but is absent in human colon carcinoma cell lines. The 6kb mRNA is by hybridization related to the human erbA gene family, but is of a different size from previously reported human erbA messages. The colonic 6kb mRNA is potentially a novel erbA transcript, differentially expressed between malignant and nonmalignant colon cell lines. Two previously cloned human erbA transcripts encode two different thyroid hormone receptors. Other erbA transcripts encode proteins with unknown functions, but which are not thyroid hormone receptors. Questions now raised are: How is the colonic erbA mRNA related to previously identified human erbA transcripts; and, how is the loss of expression of this mRNA related to colon cancer? Colon polyps are the first in a series of successive stages of colon neoplasia and can progress on to colon cancer primary tumors and colon cancer metastases. Such colon cancers are the second most common cause of cancer deaths among American adults. Previous studies have detected differing erbA products in human placenta, testis, and in breast cancer and cervical cancer cell lines, but have not investigated colon. Two different but homologous erbA genes encode by alternate splicing the previously identified erbA transcripts. Five specific questions this study will address are: 1) How is the colonic erbA 6kb mRNA related to previously identified erbA transcripts? 2) Does the colonic erbA transcript encode a colonic thyroid receptor? 3) What is the mechanism of the selective expression of this transcript in the polyp but not in the carcinoma lines? 4) Is loss of expression of the colonic erbA transcript a general marker for colon cancer? 5) Does the colonic erbA encode a suppressor whose loss directly contributes to colon cancer? To answer these questions this study will: 1) clone and sequence a colonic erbA cDNA corresponding to the 6kb mRNA; 2) examine in vitro and in vivo if this cDNA encodes a thyroid receptor with physiologic activity in colon; 3) examine if gene deletion or selective transcriptional activation account for selective expression of the erbA mRNA in the colon cell lines; 4) examine if colonic erbA mRNA is expressed in normal colonic mucosa, in colon cancer tissue, and in a collection of colon cell lines which includes lines representing each of the stages of colon neoplasia; and 5) establish if introducing the cloned colonic erbA transcript into colon cancer cell line suppresses either the tumorigenicity of the line in the nude mouse or the anchorage independent growth of the line in soft agar. These investigations will reveal significant new information about both the human erbA gene family and the biology of colon carcinogenesis.